Two winding transformers

Mandatory data required for calculations

 

Upper winding: Rated voltage kV

Downside Winding : Rated voltage kV

Winding relation: Rated power kVAr, positive sequence impedance uk%, positive sequence resistance ur%, positive sequence reactance ux%

Note! Only 2 of 3 terms are required (uk + ur or uk + ux or ur + ux)

If ur% is missing but load losses are documented, then ur% will be calculated according to: ur% = load losses / rated power * 100.

 

Recommended data

Winding relation: No load losses W, vector group

If no or faulty vector group is given, then DYN11 will be used, and a warning will appear during the calculation.

 

Tap changer

In order for the transformer to be correctly calculated when the tap changer deviates from the mid position, a component for the tap changer must be found in the correct winding. This is normally the upper side.

 

Definitions:

If the tap changer is located in the upper winding the following is valid:

Max steps. Represents the lowest voltage that can be obtained on the down side of the transformer.

Min steps. Represents the highest voltage that can be obtained on the down side of the transformer.

Rated steps. The tap changer step that will give rated output voltage for rated input voltage. Corresponds to the nominal ratio.

Set step.  A value between minimum and maximum, which represents actual setting of the tap changer.

Regulate step. A distance between two steps expressed as percentage (%) of the nominal ratio of the regulating winding.

Min voltage limit. An alternate way to set a minimum limit (kV) for transformer regulation. Is not used unless Min, Max, etc. have been set.

Max voltage limit. An alternate way to set a maximum limit (kV) for transformer regulation. Is not used unless Min, Max, etc. have been set.

Set voltage. Voltage that is wanted when calculating the position of the winding switch (default node is the bus bar on the down-side). If a LV transformer the phase voltage should be given.

Regulated node. Node ID for a point that is set to hold the actual voltage. To identify the node point the internal connection node number should be given. (Does not have to be set if the the regulated node is the bus bar).

 

Tap changer regulation

If the transformer should have a fixed voltage, the Set voltage must be defined. If the voltage should be fixed on the lower side of the transformer, the Regulated node do not have to be defined.

Otherwise it will have to be defined. Observe that for voltages < 1 kV the phase voltage should given, for other voltages the main voltage will be referred to.

No specific information is needed in the drop-down menu for Tap change regulation if the usage of fixed voltage should follow the overall settings of the calculation parameters (Regulation on / off). (Undefined or standard).

oSelect Always regulate in the drop-down box if it is desired that the transformer always shall be regulated, to keep its voltage.

oIf the transformer never should be regulated, regardless of the calculation parameters, select Never regulate in the drop-down box.

oIf the transformer should be set as the calculation parameters, select Undefined or Standard.

 

 

Alternative method to set a regulating area in steps

For certain customer specific installations, there is a possibility to set the area for the regulation with the help of the term Number of steps.

Number of steps. If Max step and Min step has not been set, but Number of steps exists, then the calculation routines will use Number of steps for the calculation.

Interpretation and conditions.

Rated step, Regulating step and Number of steps must be set. Number of steps are distributed symmetrical around rated step. In the example above the Number of steps correspond to Max steps = 3 and Min steps = -3, and rated step=mid-position = 0.

 

 

Without transformer regulation

 

If regulation has not been requested, then a new transformer ratio value will be calculated, based on the data for the tap changer.

Example 1:

 

 

New transformer ratio upper side will then be = 11 – 11 * 2.5 / 100 = 10.725

So, the input data for the calculated transformer ratio = 10.725 / 0.4 kV

This will result in a higher down side voltage.

Note, that the result term for the tap changer will in input data be described as : -1

Example 2:

 

New transformer ratio upper-side will then be = 11 + 2 * (11 * 2.5 / 100) = 11.55

So, the input data for the calculated transformer ratio = 11.55 / 0.4 kV

This will result in a voltage for the down side that will decrease in relation to the mid-position.

Note, that the result term for the tap changer will be described as: +2

With transformer regulation turned on in the calculation parameters

If regulation has been requested, then the calculation will start with nominal ratio on the transformer. Based on the data for the tap changer and parameter values, a tap changer value, if possible, be calculated to achieve the requested voltage.

Example 1:

 

 

 

The new transformer ratio upper side can then vary between:

11 + 2 * (11 *  2.5 / 100) = 11.55 kV and

11 - 2 * (11 *  2.5 / 100) = 10.45 kV.

Assume a Set voltage of  0.230 kV, and this voltage should be obtained on the busbar at regulation. The result term for the calculated tap changer value will then look like this:

Ex. = +1, = 0 or = -2.

That is, an equal sign before the value, which indicates that the value has been calculated.

If the voltage cannot be obtained within the described min/max regulated area, then the result term for the tap changer will look like this: Ex. * =2 or * = -2

where the (*) means that the regulation reached its end position and can not regulate to the requested voltage.

Example / Verification protocol of winding connecting settings

Transformer rating: 11/0.4 kV

Step = 2.5%

All calculations performed with 11,000 starting voltage.

No loads in the network.

 

 

 

 

 

 

 

 

With company-controlled "reversal flag" activated

(TAP_CHG_REVERSE=N    i  netcalc_export.conf)

 

 

 

 

 

Three-winding transformers

Three-winding transformers are created in the same way as two winding transformers, but the number of windings = 3.

The folder Transformers  > Transformer > Number of windings (further down)

 

 

Mandatory data in order for calculation to be done

Winding 1: Rated voltage kV

Winding 2: Rated voltage kV

Winding 3: Rated voltage kV

For every possible combination of windings a winding relation must be set.

Three relations

Winding 1 – 2

Winding 1 – 3

Winding 2 – 3

Necessary data for winding relations:

Rated power kVAr

Positive sequence impedance uk%

Positive sequence resistance ur%

Positive sequence reactance ux%.

Note! Only 2 out of 3 terms are needed (uk + ur or uk + ux or ur + ux)

If ur% is missing but load losses are documented, then ur% will be calculated according to: ur% = load losses / rated power * 100.

 

To set rated power

The convention to set Rated power in a three-winding transformer is as follows.

The winding relation between winding 1 and winding 2 should obtain the lowest rated power. Thus should

Relation W 1-2 Rated power = min (30000, 20000) = 20000 kVA

Relation W 1-3 Rated power = min (30000, 10000) = 10000 kVA

Relation W 2-3 Rated power = min (20000, 10000) = 10000 kVA

 

Recommended data

 

Winding relation:

No load losses W

Vector group

If vector group is missing or is wrong, then DYN11 will be used, and a warning will be shown during the calculation.

 

 

Tap changers

In order to calculate the voltage for a transformer correctly for tap changer positions that deviates from the mid position a tap changer component must be added to the upper side winding. This is also the case when calculating transformer regulations when a calculated tap changer position is obtained.

If the voltage is to be set to a fixed value then a correct voltage and regulating node point must be given set in the tap changer form. The regulated node must be set given its internal connection node id.

 

 

Specific result for a three winding transformer

Most results are the same as for two winding result terms. However there are some winding related results such as currents, voltages, etc. that differs. The substation also obtains results for each of the voltage levels.

 

 

Interpretation of result

Terms that are presented with W 1-2 refers to winding relation winding 1 – winding 2 etc.

Terms that are presented with M-3 referes to results that can’t be referenced directly to a relation or a specific winding. Since a three-winding transformer have to be modelled internally in the calculation kernel as 3 Two winding transformers with a common fictional center node. M-3 is the result from the transformer part between the center node and winding 3’s node.

 

Fixed reports

The sub station contains partial results for every voltage level. The transformer result is divided into a common result and a winding related result.

Otherwise the same documentation and calculation principles are valid for three as for for two winding transformers.